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參數資料
| 型號： | AD8309-EVAL |
| 廠商： | Analog Devices, Inc. |
| 元件分類： | 運動控制電子 |
| 英文描述： | 5 MHz.500 MHz 100 dB Demodulating Logarithmic Amplifier with Limiter Output |
| 中文描述： | 5 MHz.500兆赫和100分貝解調對數放大器輸出限幅 |
| 文件頁數： | 17/20頁 |
| 文件大小： | 311K |
| 代理商： | AD8309-EVAL |
REV. B
AD8309
–17–
Table II.
Slope
mV/dB dBV
Intercept R1
R2
k
V
R4
k
V
R5
k
V
V
OUT
(V) at
–88 dBV +12 dBV
k
V
40
50
40
50
–102
–103
–90
–90
3.92 8.87 O/C 1
1.05 9.53 O/C 1
3.92 8.87 20.5 1.05
1.05 9.53 15.4 1.07
0.56
0.75
0.08
0.1
4.56
5.75
4.08
5.10
Setting the Limiter Output Level
The limiter output is a pair of differential currents of magni-
tude, I
OUT
, from highimpedance (open-collector) sources.
These are converted to equal-amplitude voltages by supply-
referenced load resistors, R
LOAD
. The limiter output current is
set by R
LIM
, the resistor connected between Pin 9 (LMDR) and
ground depending on the application, the resulting voltage may
be used in a fully balanced or unbalanced manner. It is good
practice to retain the both resistors, whichever output mode is
used. The unbalanced, or single sided mode, is more inclined to
result in instabilities caused by the very high gain of the signal
path. If the limiter output is not needed, LMDR should be left
open with LMHI and LMLO being tied to VPS2.
The limiter output current is set by the equation:
I
OUT
= –400
mV/R
LIM
and has an absolute accuracy of
±
5%.
The voltage on each of the limiter pins will be given by:
V
LIM
=
V
S
– 400
mV
×
R
LOAD
/
R
LIM
The limiter current may be set as high as 10 mA, which requires
R
LIM
to be 40 ohms, and can be optionally increased somewhat
beyond this level. It is inadvisable, however, to use high bias
currents, since the gain of this wide bandwidth signal path is
proportional to it, and the risk of instability is elevated as R
LIM
is
reduced (recommended value is 400
).
The limiter output is specified for input levels between –78 dBV
and +9 dBV. The output of the limiter will be unstable for levels
below –78 dBV (–65 dBm).
High Output Limiter Loading
The AD8309 can generate a fairly large output power at its
differential limiter output interface. This may be coupled into a
50
grounded load using the narrow-band coupling network
following similar lines to those provided for input matching.
Alternatively, a flux-linked transformer, having a center-tapped
primary, may be used. Even higher output powers can be ob-
tained using emitter-followers. In Figure 38, the supply voltage
to the AD8309 is dropped from 5 V to about 4.2 V, by the
diode. This increases the available swing at each output to about
2 V. Taking both outputs differentially, a square wave output of
4 V p-p can be generated.
1
2
3
4
5
6
7
8
VLOG
VPS2
PADL
LMHI
LMLO
PADL
FLTR
LMDR
COM2
VPS1
PADL
INHI
INLO
PADL
COM1
ENBL
AD8309
9
10
11
14
15
16
0.1
m
F
10
V
R
LIM
RSSI
3V TO 5V
0.1
m
F
10
V
12
13
+5V
IN914
APPROX. 4.2V
R
LOAD
SET R
L
= 5*R
LIM
5V TO 3V
DIFFERENTIAL
OUTPUT = 4V pk-pk
R
LOAD
Figure 38. Increasing Limiter Output Voltage
When operating at high output power levels and high frequen-
cies, very careful attention must be paid to the issue of stability.
Oscillation is likely to be observed when the input signal level is
low, due to the extremely high gain-bandwidth product of the
AD8309 under such conditions. These oscillations will be less
evident when signal-balancing networks are used, operating at
frequencies below 200 MHz, and they will generally be fully
quenched by the signal at input levels of a few dB above the
noise floor.
1
2
3
4
5
6
7
8
VLOG
VPS2
PADL
LMHI
LMLO
PADL
FLTR
LMDR
COM2
VPS1
PADL
INHI
INLO
PADL
COM1
ENBL
AD8309
9
10
11
14
15
16
10
V
10
V
12
13
V
S
0.1
m
F
0.1
m
F
VR2
10k
V
INT
INPUT
R1
R2
VR1
2k
V
SLOPE
R3
33.2k
V
R5
R4
R6
1.96k
V
AD8031
GND
RSSI
AD8309 SUPPLY DROPPED TO 3V R
D
= (V
S
–3V)/25mA
0.1
m
F
Figure 37. Buffered RSSI Output with Slope and Intercept Adjustments
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